Energy Saving in Steam Systems

Energy saving in steam systems. Dr.G.G.Rajan / Cochin / India

Substantial quantity of savings in steam is possible by adopting the following

thumb rules. Most important operating parameters to minimize steam consumption

include using dry saturated steam having saturation temperature equal or very nearly

equal to the required process temperature in steam consuming devices such as

Steam heaters

Steam heated reboilers

Steam tracers etc.

On the contrary, superheated steam must be used in

Back pressure turbines

Extraction turbines and

Total condensing turbines.

Steam temperature or degree of super heat is an important factor that minimizes

steam consumption. Since steam temperature is related to steam pressure,

temperature parameter automatically sets the steam pressure. This would mean that

various steam heated devices require various steam pressures and super heats, depending

on the process configuration. Following table shows, how steam temperature plays an

important role in controlling steam consumption in various heating devices.

Process temperature required oC

Steam temperature oC

Corresponding steam pressure .in bar (on process temp)

Steam enthalpy Kcal/kgm

Latent heat Kcal/kgm

100 130 1.03 650.8 516.7

150 180 5.00 671.2 503.4

200 230 15.0 676.3 464.7

250 280 40.0 680.8 409.1

300 330 85.0 667.0 336.0

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Steam temperature is selected 30 oC higher than the required process temperature.

The best steam for industrial process heating is dry saturated steam, which is

neither wet not highly super heated. Wet steam reduces heat transfer while super heated

steam is not desirable for process heating because of the problems in temperature

control of the process fluids and the low heat transfer coefficient. Condensation heat

transfer is best suited for process heating applications. Available enthalpy in wet steam is

reduced depending on the moisture content present in the steam. For example, latent

heat available in a 95% dry steam, with 5% moisture shall have a latent heat of 478.2

kcal/kgm against the value of 503.4.

It is only the latent heat of steam that takes part in the process heating. Hence

it is essential that the latent heat of steam used for process heating is as high as possible.

From the above table it is obvious that lower the steam pressure, the higher is the latent

heat of steam. It may also be noted from the above table that as the steam pressure comes

down, the saturation temperature also comes down. The steam pressure for process

heating must therefore, be selected based on the required process temperature. The

saturation temperature may be selected a few degrees higher ( 10 to 20 oC) and the steam

pressure fixed accordingly.

For example, when the process temperature to be maintained is between 100 to

200 oC, it is economical to select steam pressure whose saturation temperature just

matches the process temperature plus 5 to 10 oC. Using dry saturated steam for process

heating is efficient due to it’s film heat transfer coefficient which is comparatively

higher than highly super heated steam.



Example : Furnace oil is used as a fuel in a fired heater. The furnace oil is stored in a

tank and maintained at 70 oC. The oil is heated in a steam heated exchanger to 110 oC

using a steam heater. The flow rate of furnace oil is 10 tons /hr. the specific heat of FO is

0.68. Determine the optimum steam pressure for this system and calculate the steam

rate?

Following figure shows the arrangement for heating the furnace oil. There are two

reasons for heating the furnace oil, before it goes to the burner. The primary reason is to

reduce the viscosity of the furnace oil at burner tip, so that it is atomized easily and the

secondary reason is to increase the burner efficiency and accelerate ease of combustion.

Heat required to increase FO temperature from 70 to 110 oC is given by

= Mass x Sp.heat x ∆t

= 10 x 1000 x 0.68 x ( 110-70) kcal/hr

= 272000 kcal/hr.

Steam in

Condensate

FO Tank

FO pump

Hot FO @ 110 oC to heater

70 oC

FO return

Furnace Oil Heating using dry saturated steam



Steam data:

Since the FO to be heated is from 70 oC to 110 oC, the driving force for the last three

cases in terms of average temperature is calculated as shown below. The first case is

neglected, as the driving force is too low.

Taking an average overall heat transfer coefficient of 220, 225 & 230 kcal/hr/m2/oC

respectively for the above three cases, the heat transfer area requirement for the above

cases works out to 41.77, 32.85, 27.57 m2 respectively. Steam requirement for the above

process heating is given in the last column.

It may be noted from the last column that the steam requirement in the above

cases increase with increasing steam pressure.

Steam pressure kg/cm2a

Saturation temp oC

Latent heat kcal/kg

Enthalpy in kcal/kg

1.5 110.8 532.1 643.1

2.0 119.6 526.4 646.3

2.5 126.8 521.4 648.7

3.0 132.9 517.3 650.8

Steam pressure kg/cm2a

Saturation temp oC

∆t1 ∆t2 ∆t=(∆t1+∆t)/2

Steam required in

kg/hr 2.0 119.6 49.6 9.6 29.6 516.7

2.5 126.8 56.8 16.8 36.8 521.6

3.0 132.9 62.9 22.9 42.9 525.8



Based on 8000 operating hours per year, the steam consumption works out to

4133.6, 4172.8 & 4206.4 tons respectively. While the above example refers to a single

steam heater, there will be substantial savings in steam in major process industries

comprising a number of such steam devices.

Using exhaust steam in LP service : For the LP (low pressure) process steam service,

it is economical to use exhaust steam from back pressure turbines, extraction turbines,

extraction cum condensing turbines, steam de super heaters, pressure reducers etc as

direct production of LP steam is not energy efficient. By using turbine exhaust steam,

overall thermal efficiency of the system will be high as shown in the figure1 below.

HP Steam. 100 t/hr , 40 bar , 320 oC

MP Steam, 50 t/hr, 10 bar, 220 oC

20 t/hr, 5 mmhg, ,60 0C

LP Steam 30 t/hr, 5 bar, 160 oC

~

Fig 1. Extraction turbine lay out.



Theoretical Carnot cycle efficiency of the turbine in Stage I, II & III are 16.86,

12.17 and 23.09 respectively. This is calculated by the relationship

ηcarnot = (T1 – T2) x100 /T1

where T1 & T2 are steam inlet and outlet temperatures absolute (oK) at each stage.

Theoretical power generation for the above parameters may be calculated by

enthalpy values at inlet and outlet conditions at each stage.

Though the turbine efficiency is less than 25%, the integrated overall plant

efficiency will be high, when the two extraction steam flows are used in the plant. This is

because of the fact that the entire latent heat available in these steams will be utilized for

process heating and only the condensate will be returned to the condensate recovery

system. A typical break up of energy is shown in the following table.

Energy Utilized in Turbine alone

Steam Enthalpy Kcal/kg

Flow in kg/hr

Energy input ‘000Kcal/hr

Energy output ‘000kcal/hr

Energy used ‘000kcal/hr

% utilized on input

HP (40 bar, 320 oC

720.6 100000 72060

MP (10 bar,220 oC

685.3 50000 36030 34265 1765 2.45

LP ( 5 bar ,160 oC)

658.6 30000 21618 19758 1860 2.58

Vac 25mmhg

608.63 20000 14412 12172.6 2239.4 3.11

Total 5864.4 8.14



Energy utilization in total process

Steam Enthalpy / Latent heat in Kcal/kg

Flow in kg/hr

Energy input

‘000Kcal/hr

Energy output ‘000kcal/hr

Energy used ‘000kcal/hr

% utilized on input

HP (40 bar, 320 oC

720.6

100000 72060

MP (10 bar,220 oC

685.3 / 480.3

50000 36030 10250 25780 35.78

LP ( 5 bar ,160 oC)

658.6 / 500.0

30000 21618 4758 16860 23.40

Total in process

42640 59.18

Total in tutbine section

5864.4 8.14

Overall plant

48504.4 67.32

As could be seen from the above tables, energy utilization is the highest, when the

MP and LP steams are used in the process units. The balance of 32.68 % include hot

condensate at each pressure level plus the loss of steam / condensate due to leaks,

draining etc.

Following figure2 shows the energy consumption break-up in a two stage

extraction steam turbine.



Energy Utilization Level for two stage extraction turbine

35.78

23.48.14

32.68

MP LP tutbine losses

Fig 2. Energy Utilization level for two stage extraction turbine



Economizing Steam consumption in typical back pressure / condensing steam turbines.

Steam turbines may be operated in different ways to meet the power demand. For

example, the turbine may be operated in total condensing mode or back pressure mode

depending on the steam and power demand of the process. Steam turbines used in the

process are major energy consumers. Optimizing process operating conditions can

considerably improve turbine efficiency , which in turn will significantly reduce energy

requirement. Various operating parameters affect condensing and back pressure turbine

steam consumption and efficiency.

Turbine efficiency – Significance.

The industrial sector is the largest energy consumer, accounting for about 30 % of

total energy used. With the present trend of energy prices and scarcity of hydrocarbon

resources, lowering energy requirement is a top priority. Energy conservation benefits

depend on applying certain minor or major modifications and using the modern

operational features. Energy conservation does not mean curtailing energy use alone at

the cost of industrial and economic growth. In major / mega process industries, steam

turbines are the main energy consumers. Savings achieved here will be significant, with a

better return on investment than for most other equipment.

Effect of operating conditions on steam turbines

A condensing turbine system is shown in figure 3. Turbine exhaust operating

below atmosphere, is condensed in a shell and tube exchanger called surface condenser

as shown in fig 3a.



Refer to fig 3a. Condensate flows in the shell side of the condenser and steam is

condensed by the cooling water. Vacuum in the surface condenser i.e. turbine exhaust

vacuum is controlled / maintained by vacuum ejector system of the surface condenser.

Figure 3a shows the layout of a typical surface condenser used in steam turbines.

Fig 3. Diagrammatic details of a condensing type turbine.



Fig 3a. Layout of surface Condenser

Turbines are designed for a particular operating conditions like steam inlet

pressure, steam inlet temperature and turbine exhaust pressure/ exhaust vacuum, which

affects the performance of the turbines in a significant way. Variations in these

parameters affects the steam consumption in the turbines and also the turbine efficiency.

Theoretical turbine efficiency is calculated as work done by the turbine to the

heat supplied to generate the steam.

Impact of various operating conditions on turbine performance is shown in the

table below, considering the steam conditions used in the turbine.



TABLE : Steam Consumption comparison for constant BHP

In the above referred turbines, 1 % reduction in steam consumption saves around

$ 47000 annually for condensing turbines and around $ 84000 annually in back pressure

turbine. LHV of the fuel for generating steam is considered as 10500 kcal/kg and boiler

efficiency is taken as 87 %.

Energy Balance:

Case : Condensing Turbine

Enthalpy of input steam at 40 kg/cm2a & 350 oC = 737.9 kcal / kg

Saturation temperature = 249.0 oC : Enthalpy : 669.0 kcal/kg @ saturation temperature.

Enthalpy @ outlet conditions : 647 mmhg vacuum = 113 mmhg pressure absolute

= 620.5 kcal/kgm

Saturation temperature = 53.5 oC

Net energy input = steam rate ( kg/hr) *∆ Enthalpy kcal /hr

= 27785 * (737.9 – 620.5) = 27785 * 117.4

= 3261959 kcal /hr

Carnot cycle efficiency = (350 – 53.5) x 100 /(350+273) = 47.59 %

Energy equivalent power generated = 2000 * 0.746 * 860 = 1283120 kcal /hr

Actual efficiency on energy input = 39.33 %

Condensing Type Turbine Back pressure type turbine

Steam Inlet Pressure 40 kg/cm2a Steam inlet pressure 40 kg/cm2a

Steam inlet temperature 350 oC Steam inlet temperature 350 oC

Exhaust Vacuum 657 mmhg Exhaust Pressure 4.5 kg/cm2a

Turbine rated BHP 2000 HP Turbine rated BHP 2000 HP

Steam consumption 27785 Kg Steam consumption 57960 Kg



Case : Back pressure Turbine

Enthalpy of input steam at 40 kg/cm2a & 350 oC = 737.9 kcal / kg

Enthalpy @ outlet conditions : 4.5 kg/cm2a & 148 oC = 655.2 kcal/kgm

Saturation temperature = 147.2oC

Net energy input = steam rate ( kg/hr) *∆ Enthalpy kcal /hr

= 57960 * (737.9 – 655.2) = 57960 *82.7

= 4793292 kcal /hr

Carnot cycle efficiency = (350 – 147.2) x 100 /(350+273) = 32.55 %

Energy equivalent power generated = 2000 * 0.746 * 860 = 1283120 kcal /hr

Actual efficiency on energy input = 26.77

Case : Back pressure Turbine –with exhaust steam to process.

Latent heat of steam consumed in

process = Steam rate x Latent heat (@4.5 kg/cm2a, 147.2 oC)

= 57960 x 507.1 = 29391516

Total Energy utilized = 1283120 + 29391516 = 30674636

(power) (Heat)

Overall system efficiency = 71.72 %

This example shows the practical energy efficiency improvement which may be achieved

by using the combined heat power cycle as shown above.



Steam system efficiency.

In conventional process plants, steam boilers are used to generate steam for the

process and generate power required by the process using steam turbine. The demand

scenario of power and steam (heat) is always dynamic and depends on a number of

process parameters such as through put ( capacity utilization ), feed quality, operating

severity, down stream operations etc.

A critical analysis of the process shall reveal that the overall steam system

efficiency is a function of

Boiler efficiency

Turbine efficiency

Steam transmission efficiency

Individual steam consuming device efficiency

Specific consumption of process steam in processes.

Steam losses due to leaks, blow downs, flashing venting etc.

For maintaining steam system efficiency, it is imperative to monitor all the above

factors meticulously. Boiler efficiency is a function of excess air, burner efficiency, type

of fuel used , convection and radiation losses and blow down losses.

Boiler feed water quality also plays an important role in heat transfer in the boiler

water tubes, economizer section and super heating section. Hence boiler efficiency is a

function of economizer efficiency, water evaporation efficiency in the boiler and super

heater efficiency. A simple method of determining boiler efficiency is by indirect

method in which the sum of energy losses is determined from boiler operating data as %

of energy input and subtracted from 100 to get the boiler efficiency. This method is good

enough for monitoring purposes. Figure 4 shows the various losses associated with

boiler efficiency and the table shows a typical boiler efficiency program output.



Fig 4. Energy Input / Output Diagram - Boiler

A heat balance of boiler indicates the type of heat losses which could be used

for identifying areas of improvement . A number of parameters affect the thermal

efficiency of the boiler. They are

Sensible heat in flue gases

Convection and Radiation losses.

Unburnt Combustibles in flue gases.

Unburnt combustibles in refuse

Blow Down losses etc

Sensible heat loss in flue gases:

This loss is the largest in a boiler and represents the heat carried away by the

flue gases and released to the atmosphere without doing any useful work . Hence it is

Boiler

Energy In Energy Out

Fuel Air BFW

Flue Gas Conv & Rad loss Steam Blow down



obvious that if more than the required quantity of air is used in a boiler , more will be

the loss in flue gases and the thermal efficiency of the boiler will be reduced

correspondingly.

Convection and Radiation losses :

This depends on the temperature of boiler's external surface . Quantity of heat

lost by convection and radiation is a function of shell temperature and wind velocity.

This loss occurs basically due to poor insulation and poor design characteristics .

If the refractory lining and other insulating materials are not in good condition ,

these losses will tend to increase and reduce the thermal efficiency of the boiler. Hence

it is imperative that the surface temperature at various sections should be monitored

periodically and corrective actions taken to minimize this loss.

Unburnt matter in flue gas:

When combustion is incomplete , part of the carbon present in the hydro

carbon fuel may be converted into carbon monoxide instead of carbon-di-oxide. When

the carbon content of the fuel is not fully burnt to carbon-di-oxide, there will be

substantial energy loss to the atmosphere from the flue gases ( besides atmospheric

pollution) , the loss being proportional to the amount of carbon monoxide produced .

This could be estimated by a carbon balance across the boiler taking into

consideration carbon content in fuel , carbon-di-oxide and carbon monoxide in the flue

gases and carbon content in the refuse.

Unburnt matter may show itself in the flue gas in the form of black smoke

which represents presence of carbon particles in the flue gas. It may also show itself

as Carbon monoxide in the flue gas While every kg of carbon present in the flue gas

represents a loss of 8084 kcals , every kg of Carbon partially oxidized to CO results



in a loss of 5654 kcals . This situation could be controlled with optimum excess air

input to the boiler.

Blow Down Losses:

Dissolved salts find entry to the boiler through make-up water which is

continuously fed by the Boiler Feed Water pump ( bfw) . In the boiler , there is

continuous evaporation of water into steam . This leaves behind the salts in the boiler.

Concentration of these salts , tend to increase in the boiler drum and starts precipitation

after certain concentration level .

Water from the drum should be blown down to prevent concentration of salts

beyond certain limits . Since the water in the boiler drum is at a high temperature (

equivalent to it's saturation temperature at boiler drum pressure ) , excess blow-down

will lead to loss of energy known as 'blow-down losses ' . Blow-down rate reduces the

boiler efficiency considerably as could be seen from the figure 3. Hence it is imperative

that blow-down rates are optimized ,based on the hardness levels of boiler drum water

which is a function of the operating pressure.

The program output given in the Annexure 1 shows, how various losses are

calculated for the boiler. Basis for the energy evaluation is unit weight of fuel. For

determining the quantity of energy lost in flue gas, air input quantity is calculated by the

excess oxygen leaving the stack as given by O2 monitor readings or flue gas analysis

(Orsat Analysis ).

Typical energy losses in a boiler plant is shown in fig 5. These losses are

1.Dry gas loss

2.Air moisture loss

3. Combustion moisture loss.

4.Fuel moisture loss



5.Convection and Radiation losses

6.Blow down loss.

7. Loss due to incomplete combustion of fule.

5.53

0.226.65

0.001.85

0.55

0.00

1 2 3 4 5 6 7

Dry gas loss is controlled by the excess air used in the combustion of boiler fuel.

This is the energy picked up by CO2, excess O2 and N2 from ambient temperature to

stack temperature.

Fig 5. Energy losses in steam generation using liquid fuel



Combustion moisture is controlled by the type of fuel used. High Carbon /

Hydrogen ratio of the fuel minimizes combustion moisture. But the calorific value of

high carbon fuel will be lower than lower Carbon fuel. Hence type of fuel used in the

boiler determines this loss. An optimum fuel mix (considering the cost) may be used to

minimize this loss.

In case of solid fuels, Ash content in the fuel reduces the boiler efficiency for the

same steam generation as shown in fig 6.

FIG 6. Ash content in Coal vs Efficiency (LHV)

Since the cost of coal is determined by the ash and sulfur content present in the

coal, it is possible to optimize the coal blend that will minimize the fuel cost for the same

steam generating capacity.

82

83

84

85

86

87

88

'16 % ' '21 %' '30 %' '35 %'

% eff blr



Other two controllable losses are Convection and Radiation Loss and Blow down

losses. Convection and Radiation losses may be controlled by periodic Boiler Shell

temperature survey.

Convection and Radiation losses .

For this purpose, the entire boiler shell may be divided into a number of sections

and the average temperature physically measured using a digital thermo meter or contact

thermometer. During the survey, wind velocity in the vicinity of the section may be

measured using an anemometer. Area of the section surveyed is also measured

From the shell temperature and the wind velocity, heat loss from the section is

give by standard programs in kcal / hr / m2 / oC. Multiplying this coefficient by the area

and the measured temperature, the convection and radiation loss is obtained as kcal /hr.

The sum of these values will represent total heat loss from the boiler shell due to

convection and radiation.

Annexure II gives the output of the program used to calculate the convection and

radiation loss from a boiler. Periodic monitoring of the convection and radiation loss

from boiler shall reveal the boiler refractory / shell condition.

When any hot spot develops in the boiler shell, heat loss will be more and boiler

efficiency will start dropping down. Using convection and radiation loss historic data , it

is possible to estimate the boiler efficiency deterioration as a function of time for taking

corrective action.

Fig 7 shows how the convection and radiation loss survey results are used for

developing a time dependant model.



Fig 7. Heat Loss from Boiler Sections

30.0

50.0

70.0

90.0

110.0

130.0

150.0

100 130 160 180 210 240 270 300

stream dat

heat

loss

kca

l/hr

section 1 section 2 section 3 section 4

It may be noted from the figure that convection and radiation loss from the sections 1 and

2 show an increasing tendency, while other sections are in tact. Hence necessary

corrective action may be taken at the appropriate time to restore the heat loss to the base

level.



Blow down losses :

Dissolved salts find entry to the boiler through make-up water which is

continuously fed by the Boiler Feed Water pump ( bfw) . In the boiler , there is

continuous evaporation of water into steam . This leaves behind the salts in the boiler.

Concentration of these salts , tend to increase in the boiler drum and starts precipitation

after certain concentration level .

Water from the drum should be blown down to prevent concentration of salts

beyond certain limits . Since the water in the boiler drum is at a high temperature (

equivalent to it's saturation temperature at boiler drum pressure ) , excess blow-down

will lead to loss of energy known as 'blow-down losses ' .

Blow-down rate reduces the boiler efficiency considerably as could be seen

from the figure 8. Hence it is imperative that blow-down rates are optimized, based on

the hardness levels of boiler drum water which is a function of the operating pressure.

In boiler operation practice, rate of blow down increases with steam pressure as

the scaling tendency increases with high temperature because the hardness limits are

very stringent .While figure 8 gives an estimate of % blow down on losses ,the same

may be calculated from the hardness levels of make-up water , flow rate ,steam

generation rate and the hardness level of drum water ( observed).

Model given below could be used to determine the maximum limits of TDS

(total dissolved solids) that could be tolerated in the boiler drum operating at various

pressures . The correlation developed by the author is based on American Boiler

Manufacturers' Association code of practice.

However, if the limits stipulated by the Boiler Designer is less than this value ,

the lower of the two must be taken as the tolerance limit.



Where TDS is the permissible Total Dissolved Solids in ppm at the boiler drum

and Pr is the drum pressure in psig.

Fig 8. Impact of Blow Down Rate of fuel loss.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 2 4 6 8 10 12

% blow down

% fu

el w

aste

d

14.1 kg/cm2g 10.5 kg/cm2g 7 kg/cm2g 3.5 kg/cm2g



The quantity of blow down to maintain the given status of boiler water in terms of

TDS is determined by the material balance of solids across the boiler drum as given in

the figure below.

Blow Down Rate Estimation :

For estimating the boiler drum blow down rates, we will use following

nomenclatures are used.

Let

F = feed water in t/hr

Cm = Concentration of TDS in make-up water in ppm

Cf = Concentration of TDS in feed water in ppm.

Cb = Concentration of TDS in blow-down water

m = weight fraction of make-up water in feed water.



For establishing the blow-down rate, a material balance on TDS is developed as

shown.

TDS balance: Wbd * Cb = F * m * Cm -------------------i F = Ws + Wbd ---------------------ii Therefore equation i may be written as Wbd * Cb = ( Ws + Wbd) * m * Cm -----iii

If Cf is the TDS present in the combined feed water to the boiler, above

equation may be written as

Wbd * Cb = ( Ws + Wbd) * Cf -----iv

If more than the required quantity ( i.e Wbd t/hr ) is blown down, the excess

quantity will result in lower boiler efficiency . Hence, it is imperative that boiler blow

down rate is monitored continuously for achieving high boiler efficiency. An optimal

blow down rate may be calculated taking into consideration the impact of high TDS on

poor heat transfer vs boiler efficiency.

In this section practical methods of improving the boiler efficiency was discussed

with examples. Program outputs are shown in the Annexure I & II.



Improving steam system efficiency:

Steam system efficiency may be improved by

Minimizing steam transmission losses from the steam generating plant to the

consuming sections

Minimizing steam leaks in the plant area and periodic monitoring and control of

steam leaks

Condensate recovery and

Flash steam recovery.

Steam transmission losses can be minimized by using adequate steam insulation

thickness in the steam lines and periodic monitoring of insulation efficiency by surveys.

Slightly super heated steam must be used while transferring steam to far off distances, to

minimize steam condensation in the steam line itself.

One practical way of determining the line losses is to take the steam temperature

readings at various plant battery limits and calculating the energy loss from enthalpy data,

Stefan Boltzmann and Langmuir equations. A detailed steam line insulation survey is

imperative in steam intensive sections to identify critical areas for corrective action.

Typical steam line audit survey data and observation is given in Annexure III.

Steam leak survey.

There are many methods of evaluating steal leaks from pin holes whose diameter

is known. Following table shows the steam leak from a known hole diameter and

pressure.

As the hole diameter increases, the issue of steam to atmosphere is proportional to

the square of the hole diameter, for the same header pressure. Similarly, when the

pressure inside the header increases, steam loss also tends to increase.



Steam Loss Chart

Monetary loss due to blowing traps in the system can have a dramatic effect on the

bottom line. The chart below is based a steam cost of $ 8.00/1,000 lbs. of steam

produced.

Orifice Size 5 PSI 15 PSI 30 PSI 50 PSI 75 PSI 100 PSI 125 PSI

.03 130.66 197.74 296.62 431.32 594.56 62.44 928.33

.125 520.24 790.98 1,186.46 1725.27 2,378.23 3,049.77 3,713.34

.25 2,080.96 4,745.85 3,163.90 6,901.13 9,512.94 12,199.07 14,855.30

To compute steam loss through an orifice:

24.24 x PSIA x (diameter of orifice in inches)

To convert psi to PSIA: add 14.7. Therefore, 100 PSI steam is 114.7 PSIA (absolute)

Example: Compute steam loss through a 1/8" orifice at 100 PSI steam?

24.24 x 114.7 x (.125 x .125) = 43.44

To estimate total cost of system-wide loss: Multiply the steam loss by hours of

operation, steam cost (usually between $4.00 and $14.00 per 1,000 lbs.) and by the

number of faulty traps.



Another method of identifying steam leak is to measure the plume length of the

steam length, leaking from a spot and use the following equation to assess the

approximate quantity of steam leak.

Steam Leaks are categorized into 5 types as given below.

Figure given in the next page shows the layout of a pipe line carrying steam.

Whenever any hole develops on the surface, steam will start leaking through it. The

height of plume may be measured by a steel tape or a metre scale as accurate as possible.

Another method of measuring the plume height is to hold a light metal plate

perpendicular to the direction of leak and measure the height at which the leaking steam

hits the surface with least force.

category type of leak.

a valve/trap flange leak

b valve gland leak.

c pin hole leak

d trap blowing

e Venting from vents / Desuper

Heating/Depressurizing

Stations.

stmleak = 2.5678 * EXP(1.845 * plume)

where

plume is the height of plume in meters and

stmleak is the steam leak from the measured section in kg/hr.



Steam flow

A detailed survey carried out in the plant shall reveal the type of steam leak that is

dominant over others. Quality of gasket and valve gland materials play an important role

in flange gasket and valve gland leaks.

Besides, workmanship related to proper alignment, tightening etc also tend to

increase the leak from these sections. Corrective actions may be taken to avert steam loss

due to leaks using the steam loss survey data. Annexure III shows the output of a steal

leak survey with minimum readings. A critical analysis of the type of steam leaks may be

used for improving material quality or workmanship or both based on the survey results.

H

Plume

H = plume height in metres



Conclusion :

In this paper a number of methods related to steam system management and

energy efficiency improvements have been high lighted. A number of practical examples

and cases have been covered. Since this is an area of continuous improvement and the

controlling techniques vary with cost of energy from time to time, it is imperative to

adapt to the changes as and when called for. Only by this approach , Steam system

efficiency shall be maintained at the peak.



RESULTS - BOILER EFFICIENCY

I.Fuel Data Carbon 0.870 Hydrogen 0.125 Moisture 0.000 Oxygen 0.000 Sulfur 0.005 Nitrogen 0.000 Ash 0.000 II.Observed Process parameters Boiler Duty mmkcal/h 60 % Unburnt matter in refuse 0 Amb.Temperature oC 30 Flue Gas Temp oC 180 Relative Humidity 78 Excess Air 20 High Heating Value kcal/kgm 11325 Low Heating Value kcal/kgm 10650 III.Energy Losses a.Dry Gas Loss 5.53 b.Air Moisture loss 0.22 c.Combustion Moisture Loss 6.65 d.Fuel Moisture Loss 0.00 e.Radiation Loss 1.85 f.Blow down Losses 0.55 g.Loss due to combustib 0.00 Total Loss % ( dry basis) 14.58 IV.Boiler Efficiency a.EFFICIENCY HHV basis 85.42 b.EFFICIENCY LHV basis 90.84



NOTE:

a.Air misture loss is due to moisture present in combustion air.

b.Combustion moisture is due to combustion of H2 in fuel to water.

c.Fuel moisture is due to presence of water in the fuel fired.

d.Radiation loss is due to heat loss from the exposed boiler surface.

e.Calorific Value of combustibles in refuse is taken as 7800 BTU/Lb.



Annexure II

HEAT LOSSES FROM HOT PRODUCT AND STEAM LINES USING STEFAN BOLTZMANN & LANDMUIR EQUATIONS.

Conv. Rad. Total SurfaceUnit pipe_no/ length dia in Surface insuln fluid temp Loss kcal/ Loss kcal Losses temp oC

Name code metres mm Temp oC in mm in oC /hr/m2 /hr/m2 '000 kcal/hr temp oC

crude FSS401 300 250 75 50 150.0 514.4 217.0 248.6 75.0

crude2 FSS403 350 150 65 50 130.0 375.7 160.8 152.0 65.0

Vacuum FSS401 300 100 70 60 200.0 443.9 188.3 135.1 70.0

Vac FSS401 300 200 75 80 250.0 514.4 217.0 255.7 75.0

DC 1 FSS401 300 180 80 90 300.0 586.8 246.9 291.5 80.0

DC 3 FSS401 300 200 85 90 340.0 661.0 278.2 346.6 85.0

ARU FSS401 300 190 65 80 250.0 375.7 160.8 182.4 65.0

WHB FSS401 300 180 65 80 265.0 375.7 160.8 177.2 65.0

OMS LSHS 1000 200 85 100 130.0 661.0 278.2 1216.1 85.0

OMS Bit 200 1000 150 85 100 140.0 661.0 278.2 1064.1 85.0

UTIL ut4001 2000 150 80 25 230.0 586.8 246.9 1079.5 80.0

Total Energy Loss from system '000 kcal/hr 5148.6 Note: 1.Ambient Temperature used in the program in oC : 30 2.Wind velocity for the program in km/h : 7.5



Annexure III

S T E A M L E A K S U R V E Y

UNIT TAG LEAKAGE PLUME ht STM Loss Remarks

CODE NO CATEGORY (metre) kg/h

I 1 a 2.00 102.83 I 2 a 1.50 40.88 I 3 c 0.67 8.84 I 4 b 2.67 353.97 I 5 d 1.20 23.50 I 6 a 0.96 15.09 I 7 a 0.65 8.52 I 8 b 1.12 20.28 II 9 b 1.21 23.94 II 10 c 0.76 10.44 II 11 a 0.32 4.63 II 12 a 0.21 3.78 II 13 b 1.11 19.91 III 14 b 0.32 4.63 III 15 c 0.54 6.95 III 16 a 0.96 15.09 III 17 a 1.23 24.84 III 18 a 1.08 18.83 III 19 b 0.45 5.89 III 20 b 0.23 3.93 III 21 b 0.98 15.66 III 22 a 1.05 17.82 III 23 a 0.99 15.95 IV 24 a 1.21 23.94 IV 25 b 0.91 13.76 IV 26 c 0.45 5.89 IV 27 b 0.99 15.95 IV 28 a 1.25 25.77 IV 29 a 1.99 100.95

Total Steam Leaks in kg/h 952.4613 Equivalent Fuel in kg/h 68.57722 Note : Steam Enthalpy for estimation (kcal/kg) : 720 Fuel Calorific Value in kcal/kg : 10000



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Energy Saving in Steam Systems